There is a number of models for the nonlinearities \(f\)  and \(g\) , which describe the propagation of impulses. The most relevant are the FitzHugh - Nagumo model [1]}
\({\left\lbrace \begin{array}{ll}f(u,w) = \eta [u(u-a)(u-1) + w], \\g(u,w) = bw - cu,\end{array}\right.}\) 
